1. Field of the Invention
The present invention relates to an external optical resonator for use with semiconductor lasers.
2. Description of the Prior Art
Single mode semiconductor laser transmitters having very small line widths are required for use in coherent optical transmission systems. Since the diode lasers presently availalle do not have the requisite narrow line widths, additional external measures are generally required to achieve the desired spectral properties. One such measure is to couple an external resonator to the laser for linewidth narrowing applying free beam propagation. For example, see the publication R. Wyatt, W. J. Devlin, "10 kHz. Linewidth 1.5 .mu.m InGaAs External Cavity Laser with 55 nm Tuning Range", 19 Electr. Lett. 110-112 (1983). The structure disclosed in the publication has inadequate stability and involves a considerable expense. Further, the risk of mode hopping is possible with such structure as a result of the periodic resonance structure of the Fabry-Perrot resonator used therein.
The risk of mode hopping is also present in a corresponding structure formed in fiber technology, such as disclosed in F. Favre, D. Le Guen, "Emission Frequency Stability in Single-Mode-Fibre Optical Feedback Controlled Semiconductors Lasers", 19 Electr. Lett. 663-665 (1983). Additional frequency selectivity is also lacking in the structure disclosed in the latter publication.
Additional publications disclosing narrow selection curves are L. F. Stokes, M Chodorow, H. J. Snow, "All-single-mode fibre resonator", 7 Optics Lett. 288-290 (1982) and E. J. Bachus, R. P. Braun, B. Strebel, "Polarisation-maintaining single mode fibre resonator", 19 Electr. Lett. 1027-1028 (1983). An optical waveguide having a defraction grating is disclosed in W. V. Sorin, H. J. Schaw, "A Single-Mode Fiber Evanescent Grating Reflector", Vol. LT-3, No. 5, J. of Lightwave Technology (Oct. 1985).